The invention relates to an electromagnetic brake, in particular, for an electric drive comprising at least one brake body, which has at least one permanent magnet, at least one electromagnet with an electromagnetic exciting coil, an external pole formed as an external ring, and an internal pole formed as an internal ring, wherein the brake body is locked in rotation—directly or indirectly—in particular, with the stator of a preferably electric drive, wherein the armature disc forms a magnetic circuit with the internal pole and also with the permanent magnet via an air gap, and wherein the armature disc can be drawn against the brake body by the magnetic force of the permanent magnet opposite the force of a spring and, when the exciting coil is energized, the electromagnet compensates, neutralizes, displaces, or deflects the magnetic field of the permanent magnet at least to the extent that the armature disc can be or is lifted from the brake body via the spring force.
Such electromagnetic brakes are known in various constructions and are used primarily for electric drives, where they can be imagined as stopping brakes in servomotors. The electromagnetic brake must be able to hold the drive in the electrically voltage-free state with or without play and without residual torque and it also must be able to brake the drive in case of an emergency from a certain rotational speed for a certain moment of inertia. With the help of such an electromagnetic brake, which is also designated as a permanent magnet brake, a braking moment should be able to be generated that deviates as little as possible during the service life of the brake.
For this purpose, it was previously known to provide an annular disc-shaped permanent magnet, which acts in the axial direction and whose magnetic field exits or enters perpendicular to the direction of the armature disc in the area of the internal pole and external pole, respectively. Therefore, normal magnetic forces of attraction are produced on the armature disc in the area of the internal pole and the external pole. Through a current flowing in the exciting coil, the electromagnet acts on the magnetic circuit made of the brake body and armature disc. In the non-energized state of the electrical exciting coil, there is no air gap. If current is introduced, the field of the permanent magnet is canceled by the electromagnet in the area of the poles and the armature and the restoring spring constructed, for example, as a leaf spring, pulls the armature disc away from the poles.
Such permanent magnet brakes have a fundamental disadvantage, namely that after the brake is opened, if the current in the exciting coil increases past a certain value (“repeated pull-in current”) the electromagnet significantly overcompensates for the field of the permanent magnets at the poles and leads to an undesired repeated pull-in of the armature disc against the brake body. The resulting range of the exciting current, in which the brake is opened or remains open (“air window”), should be as large as possible, so that the brake can be used safely in a wide tolerance range of exciting voltage and ambient temperature.
Due to the installation situation, here the cross section of the electromagnetic brake should not extend past the cross section of the drive, but instead, optionally, the brake should even be able to be integrated into the crankcase. In this way and due to the disc-shaped construction of the permanent magnet, the braking moment is limited because for it to increase, the radial extent of the annular disc-shaped permanent magnet would have to be increased, which is usually not possible due to the available installation space.